We are studying the architecture of chloroplasts. Photosynthesis contains two components: "light reactions and "dark " reactions. The light reactions occur on a 3-D membrane network (thykaloids) in chloroplasts and involve linear electron transport through several protein complexes, most notably photosystems II (PSII) and I (PSI). These two protein complexes are found in distinct regions of the thylakoid. Therefore, electrons must be shuttled on mobile diffusible electron carriers plastouinonone (PQ) and plasotcyanin (PC). The diffusion rates of PC and PQ are known but not the average diffusional distance because the thykaloid 3-D structure is unknown. Without a value for average diffusional distance, theorists are stymied in their desire to mathematically model photosynthetic electron transfer rates. SEM images will be used to derive values for the average diffusional distance between the two photosystems that will then be used to test theoretical models.